NEOGENE TECTONICS AND VOLCANICITY IN THE NORTH TANZANIA SECTOR OF THE GREGORY RIFT-VALLEY - CONTRASTS WITH THE KENYA SECTOR

The Neogene tectonics and volcanism in the rift area of northern Tanzania are intimately related. A major phase of late Tertiary faulting, giving rise to a broad tectonic depression, was followed by extrusion of large amounts of basaltic to trachyte magmas from large shield volcanoes. This was separated by a second major phase of faulting at about 1.2 Ma from a Late Pleistocene-Recent phase of small volume, explosive nephelinite-phonolite-carbonatite volcanism that contrasts with the earlier phase in its volume, dominant magma type and eruptive style. In both its tectonic expression and contemporaneous magmatism, the northern Tanzania province contrasts with the southern Kenya sector of the Rift Valley. The area of tectonic disturbance is considerably broader in Tanzania where ultrabasic-basic magmatism predominates. The major episodes of basaltic magmatism, representing major thermal perturbations of the mantle, have moved southwards down the rift since the mid Tertiary.